A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels. DTV signals will be transmitted in certain of the ultra-high-frequency transmission channels currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. The symbol frequency of the DTV signal is three times NTSC color subcarrier frequency, which 3.58 MHz subcarrier frequency is 455/2 times NTSC scan line rate. The pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal are positioned at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal. This causes these DTV signal components to fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal can be offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel.
The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which NTSC horizontal scan line rate is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238 million symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for DTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV display format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV display format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV display formats other than HDTV display formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. One may consider the data fields to be consecutively numbered modulo-2, with each odd-numbered data field and the succeeding even-numbered data field forming a data frame. The frame rate is 20.66 frames per second. Each data segment is of 77.3 microseconds duration. So, with the symbol rate being 10.76 MHz, there are 832 symbols per data segment. Each segment of data begins with a data-segment-synchronization (DSS) code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial data segment of each data field includes a data-field-synchronization (DFS) code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle ones of the 63-sample PN sequences in the DFS codes are transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field. The first and second logic conventions are complementary to each other (i. e., of opposite senses of polarity).
The data within data segments are trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit that is precoded. The interleaved trellis codes are subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The Reed-Solomon coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission. The Reed-Solomon coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions are made without any preceding after symbol generation. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
Receivers for VSB DTV signals are known that employ a comb filter for suppressing artifacts of co-channel NTSC interference accompanying baseband symbol coding and an intersymbol-interference suppression filter compensating for the intersymbol interference introduced by the comb filter. U.S. Pat. No. 5,087,975 issued Feb. 11, 1992 to R. W. Citta et alii and entitled "VSB HDTV TRANSMISSION SYSTEM WITH REDUCED NTSC CO-CHANNEL INTERFERENCE" concerns such a receiver. So does U.S. Pat. No. 5,748,226 issued May 5, 1998 to A. L. R. Limberg and entitled "DIGITAL TELEVISION RECEIVER WITH ADAPTIVE FILTER CIRCUITRY FOR SUPPRESSING NTSC CO-CHANNEL INTERFERENCE". FIG. 16 of the drawing of U.S. Pat. No. 5,087,975 shows the ISI-suppression filter to compensate for the precoding effects of the NTSC-rejection comb filter being located between the NTSC-rejection comb filter and the data slicer. FIG. 1 of the drawing of U.S. Pat. No. 5,748,226 shows the ISI-suppression filter to compensate for the preceding effects of the NTSC-rejection comb filter being located after the NTSC-rejection comb filter and the data slicer.
The artifacts of co-channel NTSC interference arise during the synchronous detection of the digital television signal to recover baseband symbol coding. The artifacts of the video carrier of a co-channel interfering NTSC color TV signal are at 59.75 f.sub.H, f.sub.H being the horizontal scan frequency of the NTSC signal. The artifact of the color subcarrier is at 287.25 f.sub.H, and the artifact of the unmodulated NTSC audio carrier is at 345.75 f.sub.H.
The ISI-suppression filter is a comb filter designed for matching the NTSC-rejection comb filter to cancel the intersymbol interference introduced by NTSC-rejection comb filter. The proper operation of the ISI-suppression filter depends on the intersymbol interference being of known nature. DTV signal receivers commonly include an adaptive channel equalization filter, designed to provide match-filtering for suppressing intersymbol interference arising in the transmission channel up to and including the demodulator used to recover baseband symbol coding. The filter coefficients of the adaptive channel equalization filter are commonly initialized by the training signal method, using a training signal extracted from the data field synchronizing (DFS) signals in the initial data segments of data fields. Artifacts of co-channel NTSC interference that accompany the DFS signals affect the filter coefficients of the adaptive channel equalization filter and attempt to reduce their presence. This adversely affects the match-filtering to the transmission channel that the adaptive channel equalization filter is supposed to do. So, the NTSC-rejection comb filter response is no longer match-filtered by the comb filter designed to suppress the intersymbol interference introduced by NTSC-rejection comb filter. Unsuppressed intersymbol interference accordingly raises the bit error rate associated with the conversion of baseband symbol code to error-correction-coded data, which increase in bit error rate is undesirable.
The objective of the invention is to avoid the initialization of the filter coefficients of the adaptive channel equalization filter in response to training signal being affected by artifacts of co-channel NTSC interference that accompany the DFS signals.